1. Technical Field
The present invention relates to an endoscope system.
2. Description of Related Art
Generally, to observe tissue within a body cavity, there is used an endoscope system. The endoscope system is a system which radiates, as a radiation beam, a white beam onto a portion to be observed within the body cavity, picks up a beam image due to a reflection beam from the portion to be observed using a given imaging device which is capable of imaging a two-dimensional image, and displays the thus obtained two-dimensional image on a monitor screen. A technology for controlling the radiation beam of such endoscope system is disclosed in, for example, JP-2009-056248-A, JP-2007-111151-A and JP-2005-006974-A.
In JP-2009-056248-A, there is disclosed a technology for always obtaining a radiation beam having proper beam quantity and chromaticity. Specifically, there is proposed a technology in which a drive current to be applied to a beam source is caused to change in the form of a pulse and the pulse is controlled in the number, width and amplitude thereof.
In JP-2007-111151-A, there is disclosed a technology for supplying a radiation beam onto a diseased part while controlling the heating of an endoscope leading end. Specifically, there is proposed a technology for controlling the lighting/lighting-out of a beam source in a pulse manner and also for adjusting the lighting time of the beam source and the amplitude (intensity) of the pulse.
In JP-2005-006974-A, there is disclosed a technology which, in an endoscope apparatus corresponding to multiple observation modes, can select only the observation mode to which an endoscope connected corresponds. Here, the term “observation mode” means classification such as a normal beam observation, a fluorescence observation, a narrow bandwidth beam observation and an infrared beam observation.
Here, as an imaging device which can be used in the endoscope system in order to image the two-dimensional image, there are known a CCD (Charge Coupled Device) image sensor and a CMOS (Complementary Metal-Oxide Semiconductor) image sensor. Also, as known well, the signal reading systems of the CCD image and CMOS image sensors are different from each other because they are different in structure, and the two image sensors are also different in the shutter control in photographing.
For example, a CCD image sensor of interline transfer includes a beam receiving portion, a vertical transfer portion, a horizontal transfer portion, an amplifier and the like. Specifically, it includes the vertical transfer portion which is capable of holding electric charges with respect to all pixels within the beam receiving portion. Therefore, after completion of exposure, electric charges accumulated in the respective pixels can be transferred to the vertical transfer portion respectively at the same timing. Accordingly, the timing for starting the accumulation of electric charges in the respective pixel positions of the beam receiving portion is the same in all pixels. And, the timing for ending the accumulation of the electric charges is the same in all pixels. That is, when imaging the two-dimensional image, by controlling only the image sensor, the shutter can be released at the same time for the whole of 1 frame of the two-dimensional image. This shutter control system is called a global shutter system.
On the other hand, in the case of an ordinary CMOS image sensor, from the respective pixel positions of a beam receiving unit having a two-dimensional arrangement constituted of N lines and M rows, there are read out electric charges sequentially line by line, and electric charges simultaneously accumulated are initialized. Therefore, the timing for starting the accumulation of electric charges at the respective pixel positions of the beam receiving portion is caused to shift slightly from each other every line. And, the timing for ending the charge accumulation is caused to shift slightly from each other every line. That is, when imaging the two-dimensional image, only through control of the image sensor side, the timings for releasing the shutter are caused to shift every line in the two-dimensional image, whereby the shutter cannot be released simultaneously for the whole of 1 frame. This shutter control system is called a rolling shutter system.
Therefore, in the case of an endoscope system employing an ordinary CMOS image sensor, the timings in the charge accumulation period (the time during which the shutter is substantially opened) at the respective positions of the beam receiving portion is different every scan line. Therefore, in the case that the on start timing of the beam source is adjusted in order to control the radiation beam, the radiation beam quantity varies every scan line of the two-dimensional image, thereby causing the luminance of the image to vary.
In the case that only the amplitude (beam emission intensity) of a current to be supplied to the beam source is controlled, since the radiation beam quantity is not influenced by the difference of the timings for signal reading or the like, even in an endoscope system employing an ordinary CMOS image sensor, there is no possibility that the luminance can vary every scan line. However, in the endoscope system, generally, there is required a beam quantity dynamic range of 1:9000 or more. Such broad beam quantity dynamic range cannot be realized only by controlling the amplitude of a current to be supplied to the beam source.
On the other hand, in an endoscope system employing a CCD image sensor, since the timings for signal reading and the like is not different every scan line, the on start timings of the beam source can also be adjusted in order to control a beam for radiation. Also, in an endoscope system employing a CCD image sensor, since there exists the time during which the shutter is closed simultaneously for all pixels, during this time, unnecessary radiation can be turned off, which is useful in controlling heat generation. However, in an endoscope system employing an ordinary CMOS image sensor, since the time during which the shutter is closed varies every scan line, radiation cannot be turned off during a specific period.